Henby dreyfus



Patented Oct. 25, 1932 IUNYITED sTArEs PA ENT 'OFFICEIY HENRY DREYFUS, OF LONDON, ENGLAND MANUFACTURE or ALIPHATIC non) ANHYDBIDES No Drawing. Application filed January 4, 1929, serial'Noi 330,409, and in Great Britain January 28, 1928.

This invention relates to the manufacture of aliphatic anhydrides from aliphatic acids and especially to the manufacture of acetic anhydride from acetic acid.

In Bulletin de la Socit Chimique de France Vol. XXXI (1922) pages 113-1l8 experiments are described by Peytral wherein acetic acid vapour is led through platinum tubes heated to 1150 C. In these experiments on rapid passage of the vapour,

small quantities (about 2 or 3%) of acetic anhydride were obtained, the major part of the liquid obtained by condensation of the reaction vapours consisting of acetic acid, is and the gaseous decomposition products (such as methane CO2 etc.) formed were small-in quantity; with decreased speed of passage of the vapour, however, the products of decomposition contained no anhydride and considerable quantities of gaseous products such as methane, acetylene, hydrogen, carbon dioxide were formed. It would seem from these experiments that in the case where anhydridewas produced the acetic acid vapour was passed through the tubes much too fast to allow it to attain a temperature of 1150 C. and that when owing to slower passage of the acid vapour, the vapour was subjected to higher temperatures, the reaction was more violent and the sole products of decomposition were gases such as methane, CO CO etc. My experiments have confirmed this view and I have found that when passing acetic acid vapour through platinum tubes in such manner that the vapour is allowed to attain temperatures of even 7 to 800 C. very considerable decomposition to gaseous products occurs. t is therefore evident from my experiments that when Peytral produced acetic anhydride with but negligible decomposition to gaseous products the acetic acid vapour must only have at tained a temperature below 700 to 800 (1, r and probably a temperature of from about 500 to 700 o. r

In the manufacture of acetic anhydride or other aliphatic anhydrides by subjecting acet1c acid vapour or other aliphatic acid vapour to the action of heat certain difiiculties are liable to arise when employing high temperatures and/or when performing the process for prolonged periods. Thus for instance when employing high temperatures the acid vapour often becomes decomposed with the formation inter alia of gaseous products (e.,g. methane, CO and CO) and carbon, which carbon becomes deposited in the reaction zone or,in the cases where a catalyst is employedupon the surface of the catalyst. Prolonged operation at lower temperatures appears to cause similar destructive decom? position.

I have now found that if the aliphatic acid vapour is subjected to the reaction inadmixture with carbon dioxide, and especially in admixture with relatively large quantities .of carbon dioxide, such violent decomposition to carbon and gaseous products may be largely or entirely avoided and the aliphatic anhye dride produced in a highly satisfactory manner.

According to the invention therefore I produce acetic anhydride or other aliphatic anhydride by subjecting the vapour of acetic or other aliphatic acid inadmixture with car bon dioxide thermal decomposition, i. 'e. to the action of high or relatively high temperatures. I preferably admix a substantial or relatively large quantity of carbon dioxide 7 with the aliphatic acid vapour, for instance, about 1 to 5 to 1 to 10 parts or more rela tively to each-part of aliphatic acid vapour.

It would appear thatthe admixture of the carbon dioxide (especially when substantial or relatively large quantities are employed) serves, not only as'a diluent which may diminish or prevent oxidation of the acid vapour by any air which may be pres ent, but also to oxidize and remove from the sphere of reaction any carbon which ma be formed.

he reaction may be performed at temperatures from about 250 to 700 C. and preferably at temperatures of from 300 to 600 C.

The reaction may, if desired, be performed in presence of catalysts known to promote the scission of aliphatic acids into their anhydrides, which catalysts may, if desired, be spread upon or deposited upon pumice, carborundum or other contact or filling mate'- rials. For instance, the reaction may be performed in presence of the catalysts described in my Patents Nos. 1,735,956 and 1,735 962.

In performing the invention I may for in- Stance first mix the aliphatic acid vapour with the desired quantity of carbon dioxide and pass the resulting mixture through fireclay, fused silica, copper or other tubes or other form of a paratus heated to the desired temperature, w ich tubes or apparatus may, if desired, be filled with or otherwise contain catalysts or contact materials. Or, for instance, I may pass a stream of carbon dioxide through hot or boiling acetic or other aliphatic acid and pass the resulting mixture of carbon dioxide and aliphatic acid vapour through the said heated tubes or apparatus,

- the stream of carbon dioxide being re ulated to ensure the desired composition of t e mixture of carbon dioxide and aliphatic acid vapour. It will of course be understood that the carbon dioxide may be admixed with the aliphatic acid vapour in any suitable manner and that I do not limit myself in this respect.

It is also to be understood that I do not limit myself as to the pressure to be employed in the reaction, as the reaction may be performed under ordinary atmospheric pressure, or under reduced pressure or vacuum or under pressures higher than atmospheric, such for example as 3 to 100 atmospheres or more.

The anhydride produced by the process of the invention may be separated or recovered from the reaction gases or vapours in any suitable way. In cases where the reaction is performed under pressures higher than atmos heric, the reaction gases or vapours are pre erabl not treated to separate or recover the anh d ride under such pressures but are preferably first passed through suitable apparatus (for instance one or more reducing valves) to reduce the pressure'of said gases or vapours to atmospheric pressure or other conveniently low pressure.

In order to avoid hydrolysis and consequent loss of anhydride the reaction gases or vapours are preferably not submitted to simple condensation but are treated to separate the anhydride from the water vapour present or formed in the reaction.

For instance the hot reaction gases or va-- pours may (it may be after passing throughone or more reducing valves or like apparatus) be subjected to fractional condensation, for example by leading them up through one or more fractionating columns maintained at a temperature or temperatures intermediate between the boiling points (under the condition of pressure obtaining) of the anhydride and of water, whereby the anhydride is condensed and the water passes on in vapour form.

Or, for instance, the reaction gases or vapours may (it may be after passing through one or more-reducing valves or like apparatus) be passed through one or more solvents for the anhydride which are insoluble or substantially insoluble in water and of higher boiling point than water (preferably of higher boiling point than the anhydride), such solvents being employed at temperatures intermediate between the boiling oints (under the conditions of pressure 0 taincd) of water and of the anhydride, whereby the anhydride is condensed or absorbed and the water escapes in vapour form. As examples of such solvents may be mentioned chlorbenzene, paradichlorbenzene, benzyl ether, tetrachlorcthane, par afiin oil, triacetin, phenetol, anisol, one or more cresols, and paracresylacctate.

Or, for instance, the reaction gases or vapours may (it may be after passing through one or more reducing valves or like appara tus) be subjected to condensation by the process describd in British Patent No. 303,- 7 7 2, that is to say the anhydride may be condensed from the reaction vapours whilst carrying away the water by the vapour of one or more entraining liquids. In such form of execution the reaction vapours are preferably mixed after leaving the reaction zone (and, it may be, after passing through one or more reducing valves or the like) with the vapours of the entraining liquid or liquids at a tern perature below the boiling point (under theconditions of pressure obtaining) of the anhydride Conveniently such mixing may be performed by introducing the reaction vapours (which should not be allowed to cool below the boiling point of water before becoming mixed with the vapours of the en-' training liquid or liquids) into avessel up which the vapours of the entraining liquid or liquids are caused to rise; by this means the anhydride may be substantially condensed and the water vapour carried away with the vapour of the entraining liquid or liquids Examples of entraining liquids which I may use for such method of condensation are, as indicated in the said British Patent No. 303,-" 772, benzene, carbon tetrachloride, petrol, mixtures of two or more of-such bodies, or mixtures of ether with petroleum ether; it will be understood, however, as explained in the said British Patent No. 303,7 7 2, that any other liquids chemically incl-t to-the :anhy i dride and having a high entraining capacity for Water may be employed. The liquids should preferably have a low entraining ca pacity for the anhydride.

Or, for instance, the reaction gases or vapours may (it may be after passing through one or more reducing valves or like apparatus) be subjects "l to condensation by the process described in British Patent No. 298,667, that is to say they may be subjected to condensation by leading them under the surface of anextractingliquid cooled down or otherwise kept at temperatures below (and preferably considerably below) the boiling point of water. By the term extracting liquid is meant a liquid or liquid mixture in which the anhydride is soluble and which is chemically inert to the anhydride and insoluble or substantially insoluble in water. As examples of such extracting liquids may be mentioned benzene, chloroform, and mixtures of ethyl ether or chloroform with one or more hydrocarbons such as light paraftins, gasoline, kerosene, benzol or its homologues. It is preferable to use as extracting liquids, liquids which are themselves hydrocarbons, or which contain hydrocarbons, for example, benzene or mixtures of ethyl ether or chloroform with one or more hydrocarbons such as parailins (particularly the petroleum fraction of boiling point l0 to C. termed petroleum ether), gasoline (boiling point 70 to 90 (1.), kerosene, benzol or its homologues. The following particular examples of extracting liquids are very suitable ether in admixture with petroleum ether, chloroform mixed with petroleum and/or gasoline; and mixtures of ether and petroleum ether containing about 30% to 50% petroleum ether are especially suitable.

Or, for instance, the reaction gases or vapours may (it may be after passing through one or more reducing valves or like apparatus) be treated by the process described in my U. S. Patent No. 1,817,614, that is to say they may be passed over or otherwise in contact with one or more water binding substances maintained at a temperature or temperatures below (and preferably substantially below) the temperature at which the reaction vapours are produced. By such treatment the water vapour may be substantially absorbed from the reaction vapours. In such treatment the Water binding substances are preferably maintained at temperatures above the boiling point (under the conditions of pressure obtaining) of water to avoid condensation of water and the resulting risk of loss of anhydride through hydrolysis; and for the best functioning of the treatment the water binding substances should be employed at temperatures above the boiling point of the anhydride, in which case the water can be substantially absorbed and the anhydride pass on in vapour form.

The term .water binding substance means bisulphates, pyrosulphates (especially bisulphates and pyrosulphates of the alkali and earth alkali metals) zinc chloride, calcium chloride, orthophosphoric acid, metaphosphoric acid pyrophosphoric acid and like substances which have afinity (and preferably high aftlnity) for water but excluding substances having a deleterious effect on aliphatic acids or anhydrides, such as sulphuric acid.

It will be understood that the invention is not limited as to the strength of aliphatic acid employed. The process can be performed even with the vapours of dilute acids; and besides afiording a ready means for the manufacture of anhydrides from concentrated or highly concentrated acids, it affords valuable means for producing anhydrides from waste or dilute acids, especially waste or dilute acetic acids such as result from the acetylation of cellulose or other industrial acetylation processes.

The following example serves to illustrate one form of execution of the invention, it being understood that it is given only by way of illustration and is in no way limitative.

Example A mixture of acetic acid vapour and carbon dioxide, containing about 1 to 5 parts by volume of carbon dioxide relatively to each part of acetic acid vapour is passed in a rapid stream through a copper tube heated to a temperature between 500 C. and 600 (1, the tube being filled with sodium bisulphate or sodium pyrosulphate coated upon pumice. The hot gases from the reaction zone are subjected to fractional condensation by leading them upwards through one or more fractionating columns maintained at a temperature between the boiling point of acetic anhydride and that of water, whereby the acetic anhydride is condensed from the reaction gases and the water escapes in vapour form.

What I claim and desire to secure by Letters Patent is;

1. In a process for the manufacture of an aliphatic anhydride by the thermal decomposition of an aliphatic acid, the step of avoiding violent decomposition of the aliphatic acid to carbon and gaseous products by effecting the thermal decomposition in the presence of carbondioxide.

2. In a process for the manufacture of an aliphatic anhydride by the thermal decomposition of an aliphatic acid, the step of avoiding violent decomposition of the aliphatic acid to carbon and gaseous products by efiecting the thermal decomposition in the presence of from 1 to 10 parts of carbon dioxide relatively to each part of aliphatic acid vapour.

3. In a process for the manufacture of an aliphatic anhydride by the thermal decomposition at temperatures between 250 C. and 00 C. of an aliphatic acid, the step of avoidin violent decomposition of the aliphatic acid to carbon and gaseous products by effecting the thermal decomposition in the presence of carbon dioxide.

4. In a process for the manufacture of acetic anhydride by thermal decomposition of acetic acid vapour the step of avoiding violent decomposition of the acetic acid to carbon and gaseous products by effecting the thermal decomposition in the presence of carbon dioxide.

In testimony whereof I have hereunto sub- 5 scribed my name.

HENRY DREYFUS. 

